The present invention relates to current control systems for inductive loads and, more particularly, to a time ratio controlled switching system including a controllable electric valve connected for varying the time constant of the inductive load.
Many types of operating systems require control systems having a rapid response to input command signals. Control systems having high-speed response characteristics are readily implemented using solid state electronics. In applications where the high-speed control system is utilized to control or regulate the action of a system which includes inductive reactors, the advantage attendant to a high-speed control system may be partially undermined by slow response of the controlled operating system because of the delay induced by the inductive reactance. An example of such an inductive system requiring a high-speed control system is a magnetically levitated transit vehicle in which the control system must maintain a very exact displacement between the running rail and the levitated vehicle. Motor control systems may also be limited in their response characteristics by the inductive nature of the controlled motor field.
In most operating systems the response time of the controlled system can be improved by increasing the forcing function. For example, in a motor system, a command to increase motor field current can be effected rapidly by connecting the motor field to a voltage source having a potential much larger than the potential to which the motor field must be raised in order to achieve the desired current level. Since the time required to raise the field current is proportional to the applied forcing functions and the time constant (the L/R ratio) of the field, the larger forcing function permits a rapid response. In mathematical form current is defined as EQU i=(V/R)e-(Rt/L)
where V represents the magnitude of the forcing function or voltage, R represents the resistance in the current path, L represents the inductance in the current path and t represents time. Clearly, an increase in voltage V will speed up the response time of the field. However, if the desired field current is less than the actual field current, a rapid reduction in field current cannot be effected by merely reducing the applied voltage unless the system includes negative forcing functions such as may be available in an alternating current system, provision for a negative forcing function in a direct current (d-c) system being economically undesirable.
A typical example of an operating system incorporating a high-speed control system may be found in the vehicle propulsion field in which solid state electronics have been applied to control the operation of separately excited direct current electric traction motors. In this application a time ratio controlled switch or "chopper" interconnects the motor field winding to a source of d-c potential. The chopper regulates average field winding current by cyclically switching between conducting and non-conducting states. By controlling the ratio of conducting to non-conducting time, the chopper supplies variable pulse width pulses of voltage to the field winding and the inductive reactance of the field winding tends to smooth the current produced by the voltage pulses. During the non-conducting time of the chopper, field winding current circulates through a free wheeling diode connected in inverse parallel arrangement with the winding. Field winding current can be rapidly increased by the chopper by merely increasing the chopper conduction time since the voltage available to excite the field winding is generally several multiples of the voltage at which the field winding is normally operated. However, field winding current cannot be rapidly reduced since the current decay path includes only the field winding and the free wheeling diode. The decay time constant for this path is determined by the inductance of the field and its own resistance. Thus, the high speed electronic control system is unable to rapidly reduce field winding current.
One prior art solution to this problem has been to insert resistance into the field winding circuit to change the L/R time constant. This solution, however, creates a power dissipation problem since the resistor continually absorbs energy. In one such application, the energy dissipation in the resistance element during idling of the vehicle exceeded the energy requirements for normally exciting the motor field winding.